1. Field of the Invention
This invention relates to multiport repeaters for digital data networks operating on "CSMA" or more especially "CSMA/CD" (carrier sensing multiple access with or without collision detection) protocols, such as the widely-used Ethernet protocol for local area networks. In such networks, individual stations are linked (normally via a "transceiver" or "media access unit" (MAU)) to network hardware that includes at least one "multiport repeater" (MPR) and may also include one or more than one "router" and/or one or more than one "bridge".
The links may be formed by electrical connections through wires or coaxial cables, by optical links through fibres or free space, or in any other appropriate way.
2. Description of Related Art
Data for transmission through such a network is assembled at the source location into digital packets of pre-arranged format. The format includes (among other things) a destination address segment that identifies the station intended to receive the packet and a source address segment identifying the station from which it originated, both of which are normally located near the beginning of the frame. Normally each station is allocated a unique sequence of digits (its "address") which is used to identify it whether in the destination address segment or the source address segment.
In the CSMA/CD protocol, any station that has data ready to transmit will attempt to do so provided it is not currently receiving data from the network, and if a second station begins to transmit in the short interval after the first has begun to transmit but before the second has become aware of it, a "collision" signal is generated and both stations stop transmitting and wait, before trying again, for a length of time that is either randomised or pre-allocated so as to be different for each station, so that repeated collisions between signals from the same two sources are avoided or minimised.
It is in the interest of both the speed and the capacity of the network to minimise the occurrence of data collisions, and in small basic networks data frames are distributed to all stations as rapidly as possible by at least one MPR which (or each of which) regenerates incoming signals and distributes them indiscriminately to all the stations and/or other MPR's that are directly connected to it with a time delay that is only a fraction of the length of a data-frame.
In larger networks, this approach gradually becomes unsatisfactory, as the number of stations seeking access increases and the aggregate time delay through a chain of MPR's also increases, and even in relatively small networks there may be unacceptable security implications in transmitting all the frames to every station. Some networks are therefore divided into sections linked not by MPR's but by "bridges" or "routers". The difference between a bridge and a router is irrelevant to understanding of this invention; each of these stores an incoming frame while it reads at least the destination address and compares it with stored data to determine which part(s) of the network need to receive it. Once this is done, the frame is retransmitted only to that part (or parts) of the network. Bridges and routers are complex and expensive, and they necessarily introduce a delay which will often be substantially longer than a frame length. On the other hand, they increase the capacity of the system by allowing separate control of collisions in the pre-arranged sections of the network and by reducing the overall amount of traffic on some elements of the network.